1. Field of the Invention
The present Invention relates to an optical position sensor for detecting a rotational position of a finite rotational type motor and the like. In particular, the present invention pertains to an optical position sensor that comprises a light emitting means, a light receiving means and a rotational disk positioned between the light emitting and receiving means and formed with a spiral slit, wherein a rotational angle of the rotational disk is detected in accordance with a position on the light receiving means where a light passing through the slit is received.
2. Related Art Description
The inventors of the present patent application propose a finite rotational type motor capable of high accurate light scanning In U.S. Pat. No. 5,537,874 issued on Jul. 23, 1996 and assigned to the same assignee of the present Invention. FIG. 1A shows a basic structure of this type of finite rotational motor. The finite rotational type motor 1 has an annular stator yoke 3 and a columnar rotor 2 of permanent magnet arranged coaxially inside the stator yoke 3. The stator yoke 3 is formed on it's inner circumferential surface with a pair of salient poles 3a and 3b. The torque generating mechanism of the finite rotational type motor 1 is generally the some as the respective one-phase portions of that of a typical brushles DC motor. The finite rotational type motor 1 is also provided with grooves 3c and 3d formed on end surfaces of the salient poles 3a, 3b, respectively, whereby the rotor 2 can be maintained at its initial position where the magnetic-pole direction of the rotor 2 is perpendicular to that of the stator 3. The operational angular range of the finite rotational type motor 1 is approximately between -15 and +15 degrees with respect to the initial position of the rotor 2, and an output shaft 4 connected coaxially with the rotor 2 is connected with a driven member such as a light scanning mirror and the like. The output shaft 4 of the motor 1 is provided on its opposite end with a position sensor 5 for detecting a finite rotational angular position thereof.
The position sensor 5 is an optical potentiometer which generates an analog signal in proportion to a rotational angle of the motor output shaft 4, and is provided with a rotational disk 51 fixedly connected to the motor output shaft 4. The rotational disk 51 is formed therein with a spiral slit 52 extending between an angular range which encompasses the finite rotational angular range of the motor output shaft 4. With respect to the portion of the disk 51 where the slit 52 is formed, a light emitting element 53 is arranged at one side, while a light receiving element 54 at the other side in a manner facing with each other. The light receiving element 54 is a one-dimensional semiconductor position sensing device (PSD) whose light receiving surface is arrange in the radial direction of the disk 51 for detecting a radial position of light passing through the slit, or the optical center thereof.
As shown in FIG. 1B, the curve of the slit 52 of the rotational disk 51 is defined by the following polar-coordinate expression. EQU r=ro+k.theta.
wherein k is a constant PA1 it has a linearity correction circuit for compensating an output from the light receiving means so as to establish a linearity relationship between the sensing output and the rotational angle of the rotational disk, PA1 the linearity correction circuit has a memory means for storing a corresponding table in which compensation values are assigned to respective rotational angle positions of the rotational disk and for generating one of the compensation values in responsive of the output from the light receiving means, and PA1 the compensation values are obtained by measuring an actual relationship between the rotational angle of the rotational disk and the output of the light receiving means, calculating deviation of the output of the light receiving means from a predetermined linearity relationship between the rotational angle of the rotational disk and the output of the light receiving means, and determining compensation values for eliminating the deviation of the output of the light receiving means at respective rotational angle positions of the rotational disk, wherein PA1 the linearity correction circuit retrieves a compensation value from the memory means based on the output of the light receiving means, and compensates the output of the light receiving means so as to produce the sensing output which has a linearity relationship to the rotational angle of the rotational disk.
A part of the light from the light emitting element 53 passes through the slit 52 of the disk 51 to reach the semiconductor position sensing device 54, whereby the rotational angle .theta. of the rotational disk 51 is converted into a radial position r of the light on the light receiving surface (PSD sensing area) 54a of the semiconductor position sensing device 54, the angle .theta. being in proportion to the position r.
The output currents of both ends of the semiconductor position sensing device 54 varies in an inversely proportional relationship with each other in accordance with the position r of the light on the light receiving surface 54a of the device 54. More specifically, provided that the output currents from the both ends of the device 54 be ia and ib, and that the length of the light receiving surface of the device 54 be L, the position r of the light on the light receiving surface of the device 54 is defined by the following expression. EQU R=(L/2) (ia-ib)/(ia+ib)
Thus, an output (ia-ib) can be set in proportion to the positional information r of the light by controlling the amount of light emission of the light emitting element 53 so that the current value (ia+ib) corresponding to the light energy received by the device 54 is maintained constent.
The thus constituted optical position sensor 5 must have a high linearity characteristic in which an output is accurately proportional to the rotational angle of the rotational disk 51. This linearity of the sensor 5 is adversely affected by the linearity characteristic of the device itself, accuracy of slit pattern of the rotational disk 51, assembly error of components parts, light distribution characteristics of the light emitting element 53 or the like. Hence, if error components included in the output of the sensor 5 due to these parameters are not eliminated from the output, an accurate rotational angle sensing operation of the sensor 5 cannot be guaranteed.